Magnesium orthosilicate, whose chemical formula is 2MgO--SiO.sub.2 and chemical name is Forsterite is typically used for applications requiring a high coefficient of thermal expansion. The usual procedure for manufacture of this compound is to mix pure forms of magnesia and silica in stoichiometric proportions, then calcining and milling under controlled conditions, which can be modified to meet priority specifications. Typical applications of magnesium orthosilicates are in making substrates, particularly for high frequency electronics, thick films, and ceramic-to-metal seals and high temperature bonding or joining agents.
Magnesium orthosilicate can be doped with materials such as, chromium to form Cr:Forsterite. This crystal is a new tunable laser material that fills the spectral void in the near-IR region. The tuning range for such material covers the important spectral range from 1130 to 1348 nm, which provides a minimal dispersion in optical fibers. The Cr:Forsterite laser eventually explores its niche applications for semiconductor characterization, eye-safe ranging, medical, industrial, and scientific research.
Magnesium orthosilicate, when doped with iron, forms magnesium iron silicate. Its chemical name is Olivine and the chemical formula is (MgFe).sub.2 SiO.sub.4. The special class of Olivine is Peridot, which is green in color and can be of a transparent gem variety.
In recent years, a considerable increase in the potential and actual uses of certain ceramics for structural, chemical, electrical and electronic applications has occurred because of the strength, corrosion resistance, electrical conductivity and high temperature stability characteristics of these materials. Major applications for ceramics include Si.sub.3 N.sub.4 /steel and Si.sub.3 N.sub.4 /Al joints in gas turbines and diesel engines, recuperators in heat exchangers, Si.sub.3 N.sub.4 /steel and Si.sub.3 N.sub.4 /Ti joints in fuel cells, and ZrO.sub.2 /steel joints in friction materials for bearings, bushings, brakes, clutches and other energy absorbing devices. ZrO.sub.2, Al.sub.2 O.sub.3, and mullite/steel, Si.sub.3 N.sub.4, and Al.sub.2 O.sub.3 are also wear resistant materials. TiC/steel joints have been used as materials in cutting tools and dies used in metal fabrication. SiC, Al.sub.2 O.sub.3 and BN/Al steel joints have also been used in space and military applications such as rocket nozzles, armor, missile bearings, gun barrel liners and as thermal protection barriers in space vehicles. SiC/C, Al.sub.2 O.sub.3 /Si and Al.sub.2 O.sub.3 /Cu are bonded to Al joints are used in electronic devices and other applications.
To perform effectively and efficiently for many of these applications, ceramic components chosen often must coexist with or be bonded with metallic components and form the system as a whole. Integration of ceramic-ceramic/metal hybrid parts into existing engineering designs can significantly enhance the performance of components.
The bonding or joining of ceramic parts or ceramic-metal components, however, presents a number of problems. For example, ceramic materials may differ, and ceramics and metals differ greatly in terms of modulus of elasticity, coefficient of thermal expansion, and thermal diffusivity. Accordingly, large thermally induced mechanical stresses are set up in the joint regions during bonding. In the past, this problem has been overcome only with limited success in using common techniques such as diffusion bonding, arc and oxyfuel fusion welding, brazing, soldering, and mechanical attachment techniques. Thus, while diffusion bonding has proven useful for producing joints with good elevated temperature properties, the practicality of this method is limited since it frequently requires vacuum and/or hot pressing equipment. Moreover, in the case of complicated shapes of the workpiece having non-planer mating surfaces, clamping of the workpiece to press pistons of a diffusion welding apparatus is needed to apply pressure to surfaces to be joined, which is often expensive. The maximum size of the surface area to be joined depends in such cases on the maximum force that can be brought to bear on the junction location. Alternatively, conventional fusion welding techniques create potentially critical conditions, since the material at the interface is superheated for a substantially long time, rises to accelerated reaction rates, and leads to extensive interdiffusion of species. This situation results in the formation of an entirely different microstructure with degraded mechanical and chemical properties. In general, fusion welding may only be considered for materials with low stress applications.
While soldering and brazing techniques are relatively simple to carry out and may be conducted at lower temperatures, the procedure requires elaborate surface preparation, and most importantly, the joints produced are limited to applications, which do not involve high strength or high temperature.
Other techniques, such as mechanical interlocking or electron-beam welding, have their own peculiar drawbacks. For example, electron-beam welding requires the use of a vacuum chamber. Additionally, it cannot be used with ease for dielectrics because of charge buildup on the insulating ceramics.
For a more complete description of techniques for bonding ceramic parts see the following patents:
Schroeder et al. U.S. Pat. No. 4,420,352 discloses the joining of ceramic heat exchanger parts wherein the adjacent surfaces to be joined are locally heated by RF heating.
Ebata et al. U.S. Pat. No. 4,724,020 discloses a similar process but wherein, the local heating is by high voltage torches. These disclosures require specific heating and pre-heating apparatus adding to the cost of the joining process.
Ferguson et al. U.S. Pat. No. 4,767,479 discloses a method of bonding green (unfired) ceramic casting cores. The casting cores are made up of ceramic particles and a binder. The binder in the casting cores is softened by applying a solvent and then ceramic filler particles are added to at least one of the surfaces to be joined. Thereafter, the ceramic casting cores are assembled until the solvent has evaporated. The bond strength of such joints are, however, dependent on the binder, and the use of solvents in the joining process may not be environmentally desirable.
Gat-Liquornik et al. U.S. Pat. No. 4,928,870 discloses the joining of ceramic parts by placing a metal foil or wire between the surfaces to be joined under high pressure and then subjecting the metal foil or wire to a high current for a short period of time. However, the presence of a metal at the interface may adversely affect the physical properties, e.g., corrosion resistance, and thermal conductivity of the ceramic parts.
Iwamoto et al. U.S. Pat. No. 4,952,454 discloses the joining of ceramic parts wherein a paste including metals and metal oxides in an organic binder is applied to the surfaces to be joined and then the assembly is heated to effect bonding. The organic binder upon pyrolysis can cause pores at the interface, thereby, adversely affecting the bond strength of the joined product.
Stover et al. U.S. Pat. No. 5,009,359 discloses diffusion welded joints of ceramic parts that are produced by first plasma spraying a bonding material at the seam of the mating pieces, followed by hot isostatic pressing. Such a two-step process adds to the complexity of the joining operation.
Dahotre et al. U.S. Pat. No. 5,503,703 discloses a ceramic joining process that involves using an interlayer of a mixture of two materials and reacting the materials with laser irradiation to form a thermally stable compound suitable for bonding the bodies together. The process, however, puts serious limitation on the choice of reacting materials, reaction products, reaction temperature and the ambient to avoid deleterious effects on the physical properties of the bond. Moreover, the process requires the use of a high intensity laser adding to the cost of the joining process.
It is amply clear that although the prior art is replete with numerous ceramic joining processes, there remains an important need for further innovation for an improved method of ceramic-ceramic or ceramic-metal bonding. Most importantly, there is a need for such a method which produces a bond suitable for high temperature applications, using materials of modest cost in a relatively simple and economic process.